1. Field of the Invention
The present invention relates to a porous material having micropores capable of storing and releasing heat by phase change and a preparation method thereof. More particularly, the present invention relates to a porous material having micropores capable of storing and releasing heat by phase change, which comprises a phase change material inserted into the micropores of a porous material medium, such as activated carbon or silica gel, and a preparation method thereof.
2. Description of the Prior Art
In recent years, due to concerns over the exhaustion of fossil fuels, new sources of alternative energy have been increasingly required. However, an energy source superior to petroleum in characteristics is practically impossible. In an attempt to overcome this limitation, the development of new energy storage media that store or release energy in response to change in ambient temperature is significantly needed.
In recent years, great interest has been paid to materials utilizing the latent heat of phase change materials (PCMs), which absorb and store heat in response to a rise in ambient temperature and release heat when the ambient temperature is lowered.
Typical examples of such phase change materials include paraffin waxes such as polyethylene glycol, fatty acids such as caprylic acid, inorganic salts such as hydrated inorganic salt, and so on.
Such phase change materials undergo a solid-to-liquid phase to absorb surrounding heat when the ambient temperature is higher than the melting temperature of the phase change materials, but undergo a liquid-to-solid phase change to release heat when the ambient temperature is lower than the melting temperature of the phase change materials.
FIG. 1 schematically shows paraffin wax, a conventional phase change material, encapsulated with a thin polymer film.
A method which is currently commonly used with phase change materials in practical use is a capsulation method in which the phase change material is dispersed into a liquid medium and a polymer material corresponding to the dispersed phase change material is introduced so that the phase change material is encapsulated in the polymer material.
A solvent that is used to disperse the phase change material is water, a mixture of water and methanol, or methanol.
Examples of the synthetic polymer material that is used for encapsulation include polyethylene, polypropylene, polystyrene, urea resin, polyvinyl acetate and so on.
Methods for encapsulation include physical methods such as spray drying or centrifugal force-based methods, and chemical methods such as coacervation or interfacial polymerization.
However, the method of encapsulating the phase change material using the polymer has problems in that volatile materials such as the solvent used to dissolve the polymer can cause environmental pollution problems, the method requires a very complex process and is very costly, and leaks can take place when the phase change material expands its volume in the transition from liquid to solid.
For example, when the expansion and shrinkage of the phase change material shown in FIG. 1 is repeated during its phase change, the shrinkage of the polymer material is also repeated accordingly, and the polymer material in the form of a thin film is cracked or torn when it is used for an extended period of time, resulting in the leakage of the phase change material.
In addition, because the polymer material has low heat conductivity, the functions of heat absorption and release that characterize the original phase change material are not sufficiently made use of.
Specifically, the phase change material encapsulated with the polymer according to the above-described method has low resistance to external friction or pressure, and thus is generally used in a state where it is held in a metal container having high heat conductivity.
However, in the case in which the phase change material is used in a state where it is filled in the metal container, it is difficult to construct the metal container in a micrometer size, and therefore the phase change material in the metal container has low heat transfer efficiency because the area of contact with the surrounding environment is minimal. In addition, the phase change material filled in the metal container is heavy in weight, and thus can be used only in limited applications (e.g., boilers).